Upgrading Potential of Waste Fermentation-Based C4-C5-C6 Carboxylic Acid Mixtures using Electrochemical Oxidative Decarboxylation

Abstract

Electrochemical upgrading techniques could provide a powerful tool to complement biorefineries processing waste materials, thereby making progress towards the electrification of these platforms. Among them, (non)-Kolbe electrolysis, that is, electrochemically driven oxidative decarboxylation of carboxylic acids, plays a key role as its product palette consists of alkanes (Kolbe products) as well as competing non-Kolbe products, including short-chain alkenes, alcohols, esters, ketones and aldehydes. The formation of alkanes takes place via the dimerization of radicals after the decarboxylation of carboxylic acids and requires relatively high acid concentrations (> 0.5 M) and a high current (> 200 mA cm^-2). In this work, the upgrading potential of VFA mixtures of C4-C5-C6 acids under conditions representing real waste-based fermentation effluents (lower VFA concentrations) and a suboptimal current density (100 mA cm^-2) was investigated. In addition, the effect of the initial VFA concentration ratios and the operation time (Faraday equivalents of FE = 0.25 and 1.0) on product selectivity and yields was studied. It was shown that mostly non-Kolbe products were formed – namely alcohols, esters and smaller amounts of ketones – in both experiments. An extended operation time favored ester and ketone formation, while alkanes (octane, nonane and decane) were presumably produced at the beginning of the electrolysis, since the process duration did not influence their yields. In addition, since butyric acid – the compound with the highest concentration in both model solutions – participated mostly in alcohol and ester formation, no butyrate radical dimerization (resulting in hexane) was observed. The results indicate that by not concentrating the VFAs in the fermentation effluents (via e.g. electrodialysis), the oxidative decarboxylation of VFAs at low/moderate current densities will likely result in non-Kolbe products.